Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $9.99   13.691%   $11.3577   $11.36   $11.35   $11.40  
  Purchase #2 $224.67   3.138%   $231.7201   $231.72   $231.70   $231.70  
  Purchase #3 $21.41   9.517%   $23.4476   $23.45   $23.45   $23.40  
  Purchase #4 $22.31   5.480%   $23.5326   $23.53   $23.55   $23.50  
  Purchase #5 $150.56   11.780%   $168.296   $168.30   $168.30   $168.30  
  Purchase #6 $36.62   10.587%   $40.497   $40.50   $40.50   $40.50  
  Purchase #7 $74.52   6.307%   $79.22   $79.22   $79.20   $79.20  
  7 purchase totals = $540.08       $578.0710   $578.08   $578.05   $578.00